In direct-link wireless networks, such as ad hoc networks, stations connect and communicate directly with other stations within a range without involving central access points. These stations may be, for example, portable radios. Stations with direct links to other stations have to determine when to access a data channel so as to avoid collisions and provide quality of service. One avenue of determining access to the data channel for station-to-station links requires that a centralized controller, such as an access point or a base station, determine channel access parameters including time, frequency, rate and power for each pair of stations with a direct link. On the surface, the centralized controller appears to be a simple and straight forward avenue for resolving conflicts and for ensuring that collisions do not occur when the stations are used in a system that already includes a centralized authority, as described for example in the Institute of Electrical and Electronics Engineers (IEEE) 802.16 standards or in the Long Term Evolution (LTE) standards. As used herein, “IEEE 802.16” refers to a set of IEEE Wireless LAN (WLAN) standards that govern broadband wireless access methods. Any of the IEEE standards or specifications referred to herein may be obtained at IEEE, 445 Hoes Lane, PO Box 1331, Piscataway, N.J. 08855-1331, USA. LTE is the Third Generation Partnership Project (3GPP) from the European Telecommunications Standards Institute (ETSI). LTE is used to create a high speed wireless data communications network. Any of the ETSI standards or specifications referred to herein may be obtained at 650, Route des Lucioles, 06921 Sophia-Antipolis Cedex, FRANCE.
Multiple access techniques, such as Orthogonal Frequency-Division Multiple Access (OFDMA), allow different users to share available bandwidth by allotting a fraction of a system resource to each user. For example, a receiving station in a first conversation may receive information from a transmitting station on a subset of the subcarriers on the data channel and a transmitting station in a second conversation may transmit information on a different subset of subcarriers on the data channel. However, an OFDMA system requires a complex centralized controller to resolve scheduling constraints. For example, because an OFDMA receiver must capture energy across the whole data channel band, if the receiving station is attempting to receive information from the first transmission station on subcarriers that are non-overlapping but close to the subcarriers used by the second transmitting station and if the second transmitting station is in close proximity to the receiving station, the second transmitting station may saturate or desense the receiver on the receiving station. To avoid saturating or desensing the receiver, the receiving station would have to inform the centralized controller that the second transmitting station is its neighbor so that the controller can schedule the two transmissions at different times. In order to avoid unnecessary constraints in the scheduling of transmissions, the centralized controller has to also consider the transmit power level of the second transmitting station as there is no need for the centralized controller to prevent the transmitting station with a low transmit power level from transmitting when its neighbor is receiving information. The centralized controller also has to consider a half-duplex transceiver in a time-division duplex (TDD) system because a station with a half-duplex transceiver cannot transmit and receive at the same time even if different subcarrier sets are used for the transmission and reception links.
A significant amount of resources may therefore be consumed to send parameters, such as neighbor lists, transmitter powers, and estimated ranges, from the stations to the centralized controller. Therefore, some systems allow each station to schedule its transmissions and reception. For example, the IEEE 802.11 standard provides a Distributed Coordination Function (DCF) for distributed resource negotiation. A station using the DCF randomly accesses the data channel. The station listens to the channel, measures activity on the channel, waits if the station detects channel utilization, and transmits a data frame when the channel is determined to be idle. The IEEE 802.11 standard also provides methods for reserving resources on the channel for long periods of time. The DCF, however, negotiates resources only in time. OFDMA adds a second dimension, that of frequency. The addition of a frequency component to a distributed scheduling approach greatly complicates resource negotiation and it is not obvious how to simply adapt existing negotiation protocols to handle both time and frequency.
In addition, the DCF may be an inefficient channel access method for real-time or constant bit rate traffic due to the DCF protocol's continual competition and backoff procedures. The DCF can employ any number of backoff processes, including a random backoff procedure. In random backoff, the station sets a timer to a random value chosen from a window. The station remains in the backoff phase so long as the timer has not expired. Similarly, the station could select a future frame at random and remain in backoff until that frame begins. Alternatively, the backoff duration could be a simple fixed value. Stations may set backoff timers after each successful transmission, retransmission or if the station detects that the medium is busy. The DCF resource negotiation protocol relies on the existence of idle channel time. In other words, the channel must be idle so the station's backoff timers can expire in order for the station to transmit a Request-to-Send (RTS) message. If such protocol were used in an OFDMA-slotted system in which a subset of the channel time is allocated to station-to-station communications, much of the channel time allocated to station-to-station communications would need to remain empty because stations would be running their backoff timers instead of transmitting or receiving. This would clearly increase the inefficiency of resource utilization.
Furthermore, if the DCF resource negotiation protocol is implemented in a time-slotted system, as disclosed for example in the IEEE 802.16 standard, in which a subset of the channel time is allocated to station-to-station communications, much of the channel time allocated to station-to-station communications would need to remain empty because stations would be running their backoff timers. This would clearly increase the delay in resource allocations in the direct link system. Although voice transmissions do not consume significant system resources compared to data transmissions, they require a more stringent quality of service with respect to resource allocation delays. Therefore, channel access procedure that can avoid large allocation delays while maintaining high allocation efficiency is desirable.
Accordingly, there is a need for an OFDMA channel access method and apparatus that allow stations involved in a direct connection to schedule resources on the OFDMA channel.
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The apparatus and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.